DOI: 10.1007/s11814-020-0559-5
INVITED REVIEW PAPER
eISSN: 1975-7220
INVITED REVIEW PAPER
†To whom correspondence should be addressed.
E-mail: [email protected], [email protected]
Copyright by The Korean Institute of Chemical Engineers.
Silicon and porous MWCNT composite as high capacity anode for lithium-ion batteries
Arunakumari Nulu, Venugopal Nulu, and Keun Yong Sohn†
Department of Nanoscience and Engineering, Center for Nano Manufacturing, Inje University, 197 Inje-ro, Gimhae, Gyeongnam-do 50834, Korea
(Received 7 December 2019 • Revised 5 April 2020 • Accepted 27 April 2020)
AbstractA silicon/porous multi-walled carbon nanotubes composite was synthesized using a simple method. A mixture comprising silicon nanoparticles and multi-walled carbon nanotubes was prepared by a mini ball milling method followed by annealing at low temperature. The low-temperature annealing treatment allows the aggregation of silicon nanoparticles and propels them to adhere to the outer walls of carbon nanotubes without the formation of a SiOx layer on Si nanoparticles. Mild oxidation occurring on the carbon tube walls provides additional surface defects. The obtained composite, which was studied as an anode for Li-ion batteries, exhibited excellent cyclability and superior rate capability compared with pristine silicon nanoparticles. The improved electrochemical performance of the composite can be attributed to the electrically conductive carbon tubes, easy access of the electrolyte ions into the porous nano- tube walls, and mechanical support provided by the carbon matrix. As a result, the proposed composite can sustain high discharge capacities of 1,685 mAh g1 at 1C rate after 80 cycles and 913 mAh g1 at 5C rate after 100 cycles.
Keywords: Silicon, Porous Carbon Nano-tubes, Composite, Anode Material, Li-ion Batteries
INTRODUCTION
Rechargeable lithium-ion batteries (LIBs) are regarded as among the most promising secondary batteries available in the current por- table electronics market owing to their high power, energy densi- ties, and long cell life. The high energy efficiency and high power density of LIBs may increase their applications in long range elec- tric vehicles, electric grid applications, and energy harvesting sys- tems. For practical use in the new technology of energy storage systems, LIBs should be able to maintain long-term cycle stability with high specific capacity and coulombic efficiency. Moreover, with recent advances and further improvements, LIBs can support the development of future electric vehicles. It is well known that the anode and cathode materials employed in LIBs are detrimental to the overall performance of the LIBs. Several research studies have been conducted to develop new battery materials with low cost and high performance, which can further enhance the practical appli- cations of LIBs [1-5]. The electrode materials with high specific capacity and good structural stability are required to exhibit high energy density and power density. Graphite, with a theoretical capac- ity of 372 mAh g1, is the most commonly used anode material for LIBs because of its long cycle life, availability, and low-cost produc- tion [6,7]. However, graphite also has certain drawbacks, such as its relatively low theoretical specific capacity. Hence, other anode mate- rials have been investigated to meet high energy capacity require- ments. Among them, Si has been studied extensively for its ap- plication as an anode material for LIBs due to its high theoretical
specific capacity (4,200 mAh g1), which is nearly ten-times higher than that of the graphite at low working potential (~0.4 V vs. Li/
Li+) [8-11]. However, Si experiences significant volume changes (>300%) and pulverization during lithium insertion and extraction, resulting in rapid capacity loss. Moreover, the formation of an unsta- ble solid electrolyte interface (SEI) has deterred its widespread ap- plication [12-14]. To address these limitations, comprehensive ap- proaches have been developed to overcome the significant volume changes in the Si-based electrode and its low-electronic conductiv- ity; these include the preparation of nanoscale silicon particles with carbon coating or fabrication of composites using a carbon matrix [15,16].
Owing to their high conductivity, high surface area, thermal sta- bility, and high flexibility, carbon, and its related materials such as graphene, graphite, nanotubes, and carbon paste are excellent can- didates for energy storage devices [17,18]. Due to the charge trans- fer limit, 2D and 3D scaffolds comprising hierarchically porous carbon materials and their composites are advantageous for LIBs, supercapacitors, and photovoltaics among other applications [19-23].
Some examples of these applications include the Cu-Ni Oxide@
Graphene nanocomposite for high power and durable LIBs pre- pared by Louis Perreault et al. [18]. Pedico et al. reported an rGO aerogel decorated with copper and molybdenum sulfides on car- bon fibers for high performing and wearable supercapacitors [24].
Liu et al. developed a MoO2 nanoparticles/carbon textiles cathode for a high performance flexible Li-O2 battery [25]. Luo et al. fabri- cated all-carbon electrode-endurable flexible perovskite solar cells [26]. Shih et al. developed carbon fiber penetrated sulfur crystals as a cathode material for lithium-sulfur batteries [27]. Bella et al.
developed paper-based quasi-solid dye-sensitized solar cells with cellulose-based electrodes and electrolytes [28]. Zolin et al. devel-
trochemical performance of the Si-carbon composite electrode [30- 33]. As mentioned above, the carbon and its related materials play an essential role in the development of novel types of electrode materials and improving the overall battery performance.
Multi-walled carbon nanotubes (MWCNTs) have received con- siderable attention as anode materials of LIBs owing to their abun- dant physical properties, such as high surface area, excellent electrical conductivity, thermal and mechanical properties, as well as their commercial availability [34-36]. Therefore, MWCNTs are regarded as suitable candidates for fabricating composites with silicon; this will ensure that the advantages of both Si and CNTs can be utilized to improve conductivity and cyclability while also precluding the dis- integration of electrodes [37,38]. In previous works, Si/MWCNT composites were prepared by surface functionalization of Si parti- cles followed by spray drying [39], CVD [38], acid etching of sili- con particles and then making a composite with graphene and MWCNT [40], liquid injection CVD [41], and co-axial electrospin- ning [42]. All these composites worked effectively as anodes for Li-ion battery applications. However, these methods involve highly sophisticated equipment, multi-step processes, tiresome experimen- tal parameters with extra demerits including reproducibility with gram-scale yields. In the case of silicon particles, the reported ap- proaches discussed conventional high-temperature treatment in inert gas: it is used for the fabrication of Si-carbon composites;
however, during the heat treatment process, the Si particle surface is oxidized, to some extent, to SiOx, which reduces the overall specific capacity driven by the electrode. This is a common issue during high-temperature heat treatment [37-39]. Hence, a signifi- cant facile method, which utilizes low-temperature heat treatment with minimum laboratory facilities, has to be developed. Selecting a possible efficient carbon material that is capable of making a composite with an active material using low heat is another im- portant task. To fulfill these requirements, MWCNTs have drawn our attention as a suitable carbon material to composite with Si nanoparticles.
In the present study, we proposed the fabrication of a compos- ite composed of silicon nanoparticles attached to the porous walls of an MWCNT using a simple low-temperature annealing treat- ment. This Si/MWCNT network could relieve the addition of con- ductive carbon during electrode preparation. The composite exhibits superior electrochemical performance with greater specific capacity and a much more stable cycle retention compared to that of elec- trodes prepared from pure Si nanoparticles.
Silicon nanocrystalline powder (Alfa Aesar, 50 nm), MWCNT (Hanwha Nanotech Corp. CM-100, diameter: 10 nm-14 nm), and ethanol solvent (Sigma-Aldrich, 99.5%) were purchased and used as received. The composite structures of the Si/porous MWCNT were prepared by mini milling followed by a low-temperature anneal- ing method. The step-by-step fabrication process for the Si/MWCNT composite is shown in Fig. 1. First, silicon nanopowder and MWCNT were mixed in a 9 : 1 mass ratio. Then, a small amount of ethanol was added to the mixture. The mixture was brought into a zirco- nium bowl of the mini-mill and mixed well for approximately 30 min at a speed of 30 oscillations per second. The resulting slurry was taken into an alumina crucible and calcined at 400oC for 1 h in a tube furnace under argon atmosphere. After heating, the fur- nace was cooled to room temperature. The obtained product was called SiCNT. The yield of the SiCNT was approximately 93.2%
(0.932 g) of the total original weight of the materials. This loss of 6.8% in weight could be attributed to either the annealing process or to the occurrence of error during the collection of the sample from the crucible. For comparison, an electrode was prepared with Si nanoparticles and ketjen black as the carbon additive and was called SKB. The same experimental conditions followed for SiCNT were used to develop another composite to verify the reproducibil- ity and was termed as R-SiCNT.
2. Chemical Analysis and Characterization Techniques To visualize the crystal structure of the prepared composite mate- rial, X-ray diffraction (XRD) was used. In addition, the composite material was analyzed by Raman spectroscopy. For morphological characterization, scanning electron microscopy (SEM) and trans- mission electron microscopy (TEM) were used. Thermo-gravi- metric analysis (TGA) was carried out from 25oC to 800oC in air.
3. Electrochemical Analysis
The electrochemical measurements were evaluated using 2032R- type coin cells. The working electrode was prepared by adding 80 wt% of the as-obtained SiCNT and 20 wt% of polyamide-imide (PAI) as a binder in 2-methyl N-pyrrolidone (NMP). No extra con- ductive carbon additive was added during the fabrication of the electrode. The SKB electrode was fabricated by adding 70 wt% of bare Si nanoparticles to 10 wt% of ketjen black conductive addi- tive, and 20 wt% of PAI as a binder. The components were mixed well by the mini-mill for 30 min. The resulting slurry was uniformly coated on a copper foil using a doctor blade with 15m thickness.
The electrode was dried at 80oC for 2 h to evaporate the NMP. To activate the PAI binder, the oven-dried electrode was kept in a vac-
uum oven for 3 h at 200oC. After drying, the electrode was punched (14 mm diameter). The calculated mass loading of the active mate- rial on the copper foil was ~1.2 mg cm2 with an average thickness of 150m. All coin cells were fabricated in the argon-filled glove box. Metallic lithium was employed as the counter electrode, and a polypropylene film was used as a separator. 1 M LiPF6 in eth- ylene carbonate, diethyl carbonate, and fluoroethylene carbonate with a v/v ratio of 5 : 70 : 25 was used as an electrolyte. The electro- chemical charge/discharge characteristics were measured within the 1C rate (0.1C for the 1st cycle, and 0.2C for 2nd cycle). All cells were observed within the 0.01-2.0 V window versus Li/Li+. To ana- lyze the redox behavior of the electrode, cyclic voltammetry (CV) tests were carried out between 0.01 to 2.0 V at a 0.1 mV s1 scan rate. The specific capacities of the working electrode were deter- mined based on the mass of the loaded active material.
RESULTS AND DISCUSSION
In Fig. 2(a), the XRD patterns of the Si nanoparticles, MWCNT, and the as-prepared SiCNT are shown. In the XRD patterns of Si nanoparticles, the main diffraction peaks for cubic crystalline Si are observed at 2 =28.4o, 47.4o, 56.2o, 69.2o and 79.5o (JCPDS#27- 1402) corresponding to the (111), (220), (311), (400), and (331) lat- tice planes, respectively. The XRD spectrum of MWCNT shows typical (002) and (100) peaks of carbon (see the insets of Fig. 2(a);
2 of 26o and 43o). In Fig. 2(a), the right-side inset shows the images of Si nanoparticles (pale yellow color) and MWCNT (black color) powders. The XRD peaks of the as-prepared SiCNT composite exhibit intense diffraction peaks of Si together with small peaks of carbon (25.6o and 42.6o); this implies the formation of a Si-carbon composite [40-42], which was also confirmed by the exposed black- ish-yellow–colored powder in the corresponding right-side inset image. Fig. 2(b) represents the Raman spectra of the SiCNT com- posite; the active modes located at 510 cm1 and 931 cm1 bear a resemblance to crystalline Si. Moreover, the peaks at 1,340 cm1 (D-band) and 1,585 cm1 (G-band) indicate structural defects in the carbon due to sp2 graphitic carbon, suggesting Si-carbon com- posite formation [43-45]. SEM, TEM, and TGA results of SiCNT
are shown in Fig. 3. As illustrated in Fig. 3(a), a low-magnification SEM image indicates aggregated silicon nanoparticles embedded into a dense MWCNT matrix. Fig. 3(b) shows the enlarged FESEM image of a specific portion of Fig. 3(a). The CNT porous wall defects with average pore size width lie in the range of 5-10 nm (marked with yellow-colored boxes). These porous walls were possibly intro- Fig. 2. (a) XRD patterns of Si nanoparticles, MWCNT, and SiCNT composite samples with their corresponding powder images; (b) Raman
spectra of SiCNT composite material.
Fig. 3. SEM image of (a) SiCNT composite; (b) magnified view of a specified place in image (a) (the colored boxes indicate the CNTs with porous walls). The TEM image of (c) SiCNT com- posite; the colored arrows indicate the surface defected CNTs;
(d) magnified TEM image of SiCNT. (e) HRTEM of a par- ticular silicon nanoparticle. (f) TGA curve of SiCNT.
could be attributed to the partial surface oxidation of the carbon nanotubes when annealing SiCNT at 400oC.
Fig. 4(a) shows the first three cyclic voltammetry peaks of the SiCNT electrode. In the cathodic half, an extended broad peak exists at 0.6 V versus Li/Li+, which takes into account the formation of a solid electrolyte interphase (SEI) on the surface of the electrode because of the reaction between the electrode material and elec- trolyte during its first discharge process. In the following cycles, this peak disappeared because of the formation of a stable SEI layer.
Another notable cathodic peak at 0.18 V versus Li/Li+ in the next cycles was attributed to the alloying process of LixSi. In the anodic half, the peaks at 0.35 V and 0.52 V represent the de-alloying pro- cess of LixSi to amorphous Si. The enhancement of these peaks in the subsequent cycles was due to the increased activation of the Si phase [40,46]. The electrochemical charge and discharge curves of
SKB electrode, the SiCNT electrode exhibits high initial discharge and charge capacities of about 4,541 and 3,802 mAh g1, respec- tively, with a 739 mAh g1 irreversible capacity loss through the 83.7% initial Coulombic efficiency (ICE). On the other hand, the SKB electrode delivers 3,320 mAh g1 and 2,572 mAh g1 discharge and charge capacities with 748 mAh g1 irreversible capacity loss (ICE is 77.4%). In Fig. 4(d), the cycling performance of the SiCNT and SKB electrodes at a 1C rate together with the CE values of SiCNT are shown. The ICE for SiCNT is superior to that of the SKB electrode. Moreover, it is greater than that of other well-fabri- cated Si/carbon reported composite materials [41,46-48]. The CE and discharge capacities of SiCNT and SKB electrodes for the sec- ond cycle are 91.5%, 3,792 mAh g1; and 76.1%, 2,292 mAh g1, respectively. However, the SKB electrode exhibits a decline in capac- ity up to the 35th cycle; capacity decreases from 500 to 254 mAh g1
Fig. 4. (a) Cyclic voltammograms of the first 3 cycles of SiCNT from 0.01 V to 2 V at a scan rate of 0.1 mV s1; (b) and (c) charge and dis- charge voltage profiles of SiCNT and SKB electrodes for the 1st, 2nd, 25th, and 50th cycles tested between 0.01 V and 2 V at a rate of 1C, and (d) cycling behavior of both electrodes with a Coulombic efficiency of SiCNT composite at a rate of 1C.
(fade rate of 7.15% loss/cycle for 35 cycles). Subsequently, a rapid capacity fade was observed with a discharge capacity close to dou- ble-digits (22 mAh g1 after the 80th cycle). At the end of the 80th cycle, the SiCNT electrode shows a capacity of 1,685 mAh g1, with 99.5% of CE. About 65% of capacity retention is achieved by the SiCNT electrode from the 20th cycle to the 80th cycle. This signifi- cant improvement in the cycle stability of the SiCNT, when com- pared to the SKB electrode, can be attributed to the CNT matrix within the composite, which provides not only conductive sup- port, but also excellent mechanical stability to the Si nanoparticles attached to the CNT walls. In addition, the porous walls of CNTs assist the enhanced electrochemical properties of the SiCNT com- posite. Fig. 5(a) shows the rate capability of the SiCNT and SKB electrodes at charge/discharge rates of 0.1C, 0.2C, 1C, 2C, 3C, and 5C. The SiCNT electrode exhibits reversible discharge capacities of 4,575, 3,832, 3,420, 2,893, 2,225 and 1,592 mAh g1 at 0.1, 0.2, 1, 2, 3 and 5C current densities, respectively. Even at higher current rates of 2C, 3C, and 5C, the capacity remained more than 2,000, 1,500, and 1,000 mAh g1, respectively, which is almost three-fold higher than the capacity exhibited by commercial-grade graphite anodes.
These capacity values surpass the reported Si-carbon-based hybrid electrodes with various morphologies [49-51]. The capacity was recovered to 2,878 mAh g1 when the rate was reduced to 1C after being cycled at a 5C rate, which is higher than the initially obtained capacities at the 2C rate. As regards the SKB electrode, when the current rate was increased from 0.1C to 5C, the capacities were drastically decreased and almost reached zero at 5C. The poor cycling and rate performance can be attributed to the pulverization of Si nanoparticles during the crumbling of the electrode material.
The formation of the Li-Si alloy during cycling induces significant volume expansion, which can lead to the creation of micro-cracks on the electrode surface, resulting in loss of contact with the cur- rent collector, and this consequently results in poor cycle reten- tion. This result suggests that the combination of Si nanoparticles with CNTs greatly enhanced the cycling capability of the compos- ite SiCNT by providing an electrically conductive network and facile strain relaxation afforded by CNTs. Further, the superior mechanical strength of CNTs can help anchor active Si nanoparti- cles and prevent aggregation during cycling. In Fig. 5(b), we cycled the SiCNT cells at higher rates of between 2 V and 0.01 V. Initially,
at high current rates of 3C and 5C, the discharge capacity of SiC- NTs exhibited high values of 3,276 mAh g1 and 3,648 mAh g1, respectively; moreover, they remained at 968 mAh g1 and 913 mAh g1 at 3C and 5C, respectively, after 100 cycles. SiCNTs also exhibited good CEs of 86.2% at 3C and 84.6% at 5C for their ini- tial cycles and ended at high efficiency of greater than 98%. After a total of 100 cycles, the cells showed capacity retention of 67%
(versus the 25th cycle) at 3C and 80% (versus the 25th cycle) at 5C, respectively.
The TEM image of the electrode, which was cycled at 3C for 100 cycles, is shown in Fig. 6(a) and Fig. 6(b). The image indicates the amorphous phase and aggregated nanoparticles of Si firmly connected to the fragmented CNT segments (marked with col- ored arrows in Figs. 6(a), (b), and (c)) with the rugged SEI layer (spotted with black arrows in Fig. 6(b)). Though the electrode cycled such high current density, there was inconsequential structural damage and the SiCNTs exhibited good structural integrity. More importantly, some portion of the amorphous silicon was smeared into the irregular defected pores of CNTs (marked with yellow- colored arrows in Fig. 6(c)) and remained firmly connected to the conductive CNT walls during cycling, and thereby proved to be highly resistant to the volume changes associated with Si expan- sion. Therefore, the superior performance of the proposed electrode can be attributed to the increased Si volume and the mechanical integrity of the Si/CNT/SEI layer. Fig. 6(d) shows a schematic dia- gram of the Si/CNT vs. Li/Li+ half-cell, where porous CNT pro- vides a conductive network for Li-ion reactions with Si particles and CNT walls; after 100 cycles, image Fig. 6(c) demonstrates struc- tural stability. To assess the reproducibility of the proposed system, TEM images of the composite R-SiCNT were taken (Fig. 7(a) and (b)). From the images the porous MWCNTs and composite forma- tion indicated with yellow arrows can be observed, and it is clear that these structures are similar to that of Si/porous MWCNT struc- tures depicted in Fig. 3(c) and (d), indicating the possibility of repro- ducibility of the system. The final yield of this composite was 92.7%.
When compared with the SiCNT sample, the error in reproduc- ibility was 0.54%, which shows our method is reproducible with repeated structures. All these attributes of the SiCNT electrode render it an easily reproducible and superior anode electrode mate- rial for LIBs. Moreover, the SiCNT nanocomposite obtained using Fig. 5. (a) Rate capability of SiCNT and SKB electrodes at different current densities; (b) cycling behavior and Coulombic efficiency of
SiCNT composite at 3C and 5C.
this method exhibits competitive and better cycle performance than that reported on a variety of Si/CNT nanocomposites (Table 1).
Our proposed synthesis method, without the use of any sophisti- cated equipment and with low-temperature annealing, simple experi- mental conditions, and excellent reproducibility, is suitable for producing efficient carbon composite materials at a low cost for a wide variety of energy storage applications.
CONCLUSION
A porous SiCNT hybrid nanocomposite comprising silicon nano- particles was successfully integrated with a porous MWCNTs matrix.
The hybrid nanocomposite was synthesized through a simple two- step process involving mini-milling followed by low-temperature annealing. When used as a Li-ion battery anode, the prepared Fig. 6. (a) TEM image of SiCNT composite after 100 cycles at 3C; the small orange-colored arrows indicate CNT segments; (b) magnified view of (a); (c) amorphous silicon smeared into CNT segments (marked with yellow arrows) integrated network. (d) schematic dia- gram of Si/porous CNT reactions with the electrolyte and after cycling depicts image 6(c) composed of portions of CNT with amor- phous Si.
Fig. 7. (a) and (b) TEM images of R-SiCNT sample (formation of defected CNTs and SiCNT composite shown with yellow arrows).
composite electrode exhibited high reversible capacity, excellent cycle stability, and rate capability. The notable improvements in electrochemical properties can be attributed to the collective out- come of both silicon nanoparticles and porous CNTs. The CNTs with defective porous walls appearing in the composite provide a flexible conductive matrix that can endure severe volume changes in the Si nanoparticles which are brought upon by lithium-ion reac- tions and prevent electrode pulverization. Moreover, the excellent cycling performance of the proposed composite, even at high cur- rent densities of 3C and 5C, can be attributed to the robust adher- ence of amorphous silicon onto the flexible porous CNT segments.
This excellent performance makes the fabricated composite a prom- ising anode material for lithium-ion batteries. These results, together with the simplicity of the proposed approach, provide an inexpen- sive and facile synthesis strategy for the development of efficient electrode materials for energy storage applications.
ACKNOWLEDGEMENTS
This research was partly funded by grants (NRF-2015R1D1A1 A01059983 and NRF-2018R1D1A1B07044026) from the Basic Science Research Program through the National Research Foun- dation of Korea (NRF) which is funded by the Ministry of Educa- tion. We thank them for their financial support.
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Table 1. Comparison of electrochemical performance of different Si/CNT-based nanocomposites with our results Composite materials Current density (mA g1)
or 1C=~3.6 Ag1
Reversible specific
capacity (mAh g1) References Multi-walled carbon nanotube-embedded sil-
icon nanoparticle films
840, 100 cycles 1,510 39
Nanostructured hybrid silicon/carbon nano- tubes heterostructures
2.5C, 5 cycles ~1,000 41
Scribble multi-walled carbon nanotubes-sili- con nanocomposite
300, 50 cycles and 2,000, 5 cycles
2,250 and 1,550 47
Light-weight free-standing carbon nanotubes- silicon films
360, 5 cycles and 1,200, 50 cycles.
~2,400 and ~1,900 51
Si-MWNT nanocomposite 500, 10 cycles ~800 52
Single-walled carbon nanotubes/silicon com- posites
50, 50 cycles ~1,400 53
Silicon/porous multi-walled carbon nanotubes composite
1C, 80 cycles and 5C, 100 cycles
2,000 and 1,100 This work
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